Petroleum engineers generally estimate an amount of hydrocarbons stored in a geological formation based on well logging data, including resistivity measurements, from the formation. Logging-while-drilling (“LWD”) sensors provide resistivity measurements of a geological formation as a wellbore is being drilled through the geological formation. Wireline (“WL”) sensors provide resistivity measurements of the geological formation after the wellbore has been drilled through the geological formation. For the purposes of this application, a wireline or WL sensor should be understood to include any sensor deployed in a pre-drilled borehole, such as those deployed on slick lines or coiled tubing. After the wellbore has been drilled, mud and other material can penetrate into the geological formation adjacent the wellbore, which is called an “invaded” zone. The invaded zone can distort resistivity measurements associated with the geological formation.
Because the WL resistivity measurements are strongly affected by several environmental effects, such as borehole rugosity, invasion, and shoulder-bed effects from adjacent layers, special modeling and inversion based interpretation techniques have been utilized to extract information about the formation properties from the WL resistivity measurements. Similarly, because the LWD resistivity measurements are also strongly affected by the foregoing environmental effects, special modeling and inversion based interpretation techniques have been utilized to extract information about the formation properties from the LWD resistivity measurements.
A problem associated with the current interpretation techniques, however, is that the techniques generate a model of a geological formation that may represent an equivalent solution of an inverse problem and therefore does not accurately represent an actual resistivity model of the geological formation.
Accordingly, there is a need for an improved system and method for determining a more accurate model of a geological formation using well logging data from both a LWD sensor and a WL sensor.